1. Field of the Invention
This invention relates to MOS transistors. More particularly, this invention relates to such transistors arranged to avoid the degrading effects of "hot carrier" injection into the gate oxide.
2. Description of the Prior Art
The degrading effects of "hot carrier" injection into the gate oxide of a MOSFET have been known for some time. Such "hot carriers" typically are electrons, and have energies higher than thermal energy. The adverse effects comprise parametric instabilities such as changes in threshold voltage (V.sub.T), as well as changes in sub-threshold slope and transconductance. With the ever-smaller dimensions resulting from progressive advances in miniaturization, and particularly when operating the device at desired higher voltages, these adverse effects can result in substantial reduction of the predicted useful or stable lifetime of a MOSFET device, e.g., from 10 years to a few days.
The observed effects seemingly are due at least in part to hot electrons becoming trapped in the SiO.sub.2 under the FET gate, thereby raising the threshold voltage required to obtain a specified current, or reducing the current for a given gate bias voltage. Reduced dimensions exacerbate these effects because the electric fields which create the energetic carriers are correspondingly increased.
An increase in substrate current (Isub) correlates with increased degradation due to hot carrier injection, and thus has been used as an indicator of the extent of the problem. Such substrate current in NMOS devices are holes, and add to the drain current so as to produce a non-linear output characteristic. The effect of hot carrier injection on device lifetime is determined by accelerated testing, as by placing the test device under increased drain voltage conditions. Lifetime can be defined as the time to reach a 10 mV shift in device threshold voltage (V.sub.T), and is extrapolated from a plot of lifetime (.tau.) vs. Isub under DC stress.
Various proposals have been made for solving this problem. For example, it has been proposed to reduce the dopant (impurity) concentration in the drain region. However, none of the prior proposals have been fully effective in adequately minimizing the degradation of performance due to hot carrier injection, particularly for applications in the field of analog circuits.